The present invention relates in general to vehicular drive axle systems and in particular to an improved four wheel drive system including means for disengaging two of the four driven wheels from the power generation system of the vehicle.
Four wheel drive systems for vehicles are becoming increasingly common. Such systems are usually provided with a primary pair of driving wheels, which are constantly engaged with the power generation system of the vehicle, and a secondary pair of driving wheels, which are typically only engaged to the power generation system of the vehicle at selected times. When so engaged, each of the four wheels is capable of driving the vehicle across a surface upon which the vehicle is to be operated. The primary pair of driving wheels can be either the front or rear wheels, depending upon the particular design of the vehicle. In either event, some mechanism is provided for selectively connecting and disconnecting the secondary pair of driving wheels from the power generation system so as to permit the vehicle to be operated in either the four wheel drive or two wheel drive mode.
In most four wheeled vehicles, the steering system is designed such that the turning radii of all four wheels converge at a single point. Since the four wheels are spaced apart from one another, their turning radii are all different. When the vehicle is proceeding through a turn, a different distance is traveled by each wheel in the same period of time and, consequently all four wheels rotate at different speeds. Thus, the inside wheels cut in more tightly than their respective outside wheels, and the rear wheels turn more sharply than the front wheels. The relatively slower speed of an inside wheel with respect to the relatively faster speed of its respective outside wheel is typically compensated for by a conventional intra-axle differential assembly or an overrunning clutch assembly on the axle, which permits the outside wheel to rotate faster than the inside wheel. In conventional four wheel drive systems, an inter-axle differential assembly can be provided to compensate for the variation between the relatively fast speed of the front axle and the relatively slow speed of the rear axle. The input to the inter-axle differential assembly is connected to the power generation system of the vehicle, while the outputs therefrom are connected to the front and rear axles. The inter-axle differential assembly suffers from the same problem as the intra-axle differential assembly, namely, that no power will be transmitted to one of the outputs if the other of the outputs rotates freely, such as when one wheel loses traction on a slippery surface. Furthermore, because the front and rear axles are driven at a constant speed relative to one another by the power generation system of the vehicle, undesirable wear can occur on the tires of the wheels or, more seriously, torsional forces may be created between the front and rear axles when the vehicle is turned on a non-slippery surface. Thus, as mentioned above, conventional four wheel drive systems include some means for disconnecting the secondary pair of drive wheels from the power generation system of the vehicle to prevent this from occurring.